A plug for sealing an opening of a vial container is required to have a number of quality performances e.g., sealing performance, gas barrier performance, chemical resistance and needlestick resistance. Rubber plugs having excellent elastic deformation capabilities have been widely used as plugs which satisfy these required performances.
In general, a rubber plug is formed so as to have a thick discoid cap portion and a thick cylindrical leg portion. The cylindrical leg portion has a diameter which is smaller than that of the cap portion and protrudes from the bottom surface of the cap portion. The lower surface of the flange of the discoid cap portion can be in close contact with the end surface of the opening of the vial container by driving the cylindrical leg portion into the inner periphery of the opening of the vial container.
Among these types of rubber plugs for vial containers, a rubber plug for sealing an opening of a vial container which contains medicinal chemicals needs to have quality properties which comply with a test method for an infusion rubber plug which is described in the 15th revised Japanese Pharmacopoeia. In particular, such a rubber plug needs to pass an eluted substance test for one hour at 121° C. using a high pressure steam sterilizer which requires a high heat resistance.
As a material for a rubber plug for a vial container which meets the requirement, synthetic rubber like butyl rubber or isoprene rubber, styrene thermoplastic elastomer like SEBS and thermoplastic elastomer which comprises polyisobutylene or polybutadiene in major proportions have been conventionally utilized.
In general, the surface of rubber materials is adhesive. The rubber plugs made of this kind of rubber materials may happen to adhere each other and may cause troubles when a number of rubber plugs are handled together in a single production line.
It is proposed to form projecting portions in a texture pattern on at least part of the surface of a rubber plug for a vial container in order to prevent rubber plugs for vial containers from adhering each other. See Patent Document 1, for example. It is also proposed to deposit fluoro-rubber vulcanization coating on part of the surface or entire surface of a rubber plug for a vial container. See Patent Document 2, for example. It is further proposed to coat the surface of a rubber plug for a vial container with a thermo-plastics film. See Patent Document 3, for example.    Patent Document 1: Japanese Laid-open Patent Application No. Hei 10-94581    Patent Document 2: Japanese Laid-open Utility Model Application No. Sho 55-47850    Patent Document 3: Japanese Laid-open Patent Application No. 2002-209975